The number of votes to get in the top 3 in the last few years have been....
2014 -
Lewis Hamilton - 209,920
Rory McIlroy - 123,745
Jo Pavey - 99,913
2013 -
Andy Murray - 401,470
Leigh Halfpenny - 65,913
AP McCoy - 57,854
2012 -
Bradley Wiggins - 492,064
Jessica Ennis - 372,765
Andy Murray - 230,444
(Higher than usual this year probably down to the massive Olympic year)
2011 -
Mark Cavendish - 160,152
Darren Clarke - 42,188
Mo Farah - 29,780
2010 -
AP McCoy - 293,152
Phil Taylor - 72,095
Jessica Ennis - 62,953
Apart from the unusually high 2012 because of the massive success of the Olympics then in most of those years 3rd place is very do-able. The advantage it looks like Sinfield will have is mass voting. More people will vote for the likes of Murray or Ennis-Hill, but they'll probably vote once or twice. Sinfield it seems like will have a significant amount of people voting 10/15 times if not more for him (and at only 15p a go then why not?)
Really depends who and how many tune in to watch the show and then bother to vote. As shown above some years (particularly 2011) seemingly people weren't as fussed about picking up the phone. Often people will say they think "so and so" should win but many don't actually go ahead and bother to vote.